This invention relates to time division telephone switching systems and more particularly to the implementation of key telephone service in time division private branch exchanges.
Time division PBXs conventionally employ solid state crosspoints which are operated for a fraction of a second called a time slot and which provide for the transmission of voice samples without, however, the ability to provide a d.c. or metallic conduction path between the subscriber's station port circuit and the trunk port circuit. The sleeve lead conventionally found in space division switching systems is also not normally present. This normally causes no undue problems when the switching network is simply handling ordinary telephone sets. However, when a key telephone set having a hold button and one or more illuminated line pick-up keys is served a problem arises which has been discussed in R. M. Averill, Jr. U.S. Pat. No. 3,420,961 issued Jan. 7, 1969. Briefly, when a call involving such a set is placed on hold by the key set user a holding bridge is inserted across the tip and ring conductors. The holding bridge is physically located in a key telephone unit that is interposed between the telephone set and the line circuit appearance in the switching network. A typical key telephone unit of the so-called 400-series type is shown in R. E. Barbato et al U.S. Pat. No. 3,436,488 issued Apr. 1, 1969.
The operating of the hold button at the key telephone set causes an A relay in the 400-series equipment to release and thereby insert the winding of a loop-monitoring L-relay in series with the holding bridge. When the station user removes the hold condition by operating the pick-up key for the line, the A relay is reoperated, the L-relay is released, and the holding bridge is removed. Other contacts of the A relay control other relays that steer the key button lamp illumination potential between a source of steady current when the line is picked up to a source of wink lamp illumination current when the line is placed on hold.
When such a holding bridge arrangement is employed in a conventional step by step or crossbar PBX and the call is abandoned by the party at the remote end of the trunk, the on-hook state of the trunk is reflected by a change in the d.c. state of the line circuit and the holding bridge is removed and the lamp illumination state changed to reflect the true state of affairs. When such a holding brige arrangement is employed with a time division PBX however the holding bridge is not released and the lamp illumination continues at the wink rate giving the station user the erroneous impression that the remote party is still being held.
Heretofore the problem has either been ignored or has required the use of the rather complicated special scan rate distinguishing circuitry disclosed in the above mentioned R. M. Averill, Jr. patent. In that patent, the line circuit was required to be provided with equipment to distinguish between the scanning rates present for held lines and the interruption of scanning by the processor when it determined that the call had been abandoned. It would be advantageous to provide a somewhat less complex arrangement at the line circuit for dealing with the changes occurring on the trunk side of the PBX network and for reflecting those changes at the station side.
It would also be advantageous to provide a means for releasing the hold bridge at a line circuit when the call is abandoned at the remote end of the trunk, or by the other party in an intercom call.